In a wireless communication system, a technique of reducing multi-user interference in the same system or interference waves with different modulation methods in the outside of the same system is required to improve a system capacity or to stabilize a system operation. A system which uses multiple antennas is proposed as a method of realizing an interference wave suppression technique.
In order to suppress the multi-user interference in the same system, various methods such as, spatial division multiple access (SDMA) as a multiple access method, a method of performing beamforming by a transmitter, and successive interference canceller (SIC) on a reception side are researched and put into practical use. Among the methods, a technique which uses multiple antennas is a mainstream, and multiple-input multiple-output (MIMO) technique can also transmit and receive multiple streams using the same frequency at the same time. That is, since path separation is possible, such technique is a technique of suppressing interference in a broad sense. Since it is possible to get a location of a user, to manage resource block allocation, or to insert a known signal such as a pilot signal into a radio frame of a transmission signal in advance in the same system, it is possible to easily suppress the interference wave.
Meanwhile, it is not easy to suppress interference waves in the outside of the same system. In a frequency band such as an industry science medical (ISM) band in which an interference wave such as an illegal radio wave and a plurality of communication systems exist together, a modulation method, a bandwidth, and the like are different from each other, and thus, it is impossible to grasp and manage other systems. Under such a circumstance, there is a null control technique of using multiple antennas as illustrated in FIG. 1 as an interference wave suppression technique. FIG. 1 illustrates an example in which the number of antennas is two, and an interval between the antennas is set to a half wavelength of a system of the antennas. By synthesizing amplitudes of two received signals to be equal and phases thereof to be opposite in order to cancel an interference wave, it is possible to extract only a desirable wave from which the interference wave is removed. Various algorithms are proposed as a method of suppressing an interference wave and extracting a desirable wave from two received signals, that is, a method of increasing a desired to undesired (D/U) signal ratio.